Tire tread patterns are provided with numerous elements such as ribs and blocks, the elements being separated by circumferential and/or transverse grooves. The grooves provide means for water evacuation and form the biting edges of the tread elements.
When a tire is new, the tread has a maximum tread height. This initial height may vary depending upon the intended use of the tire; a winter tire has an increased tread depth in comparison to an all season tire. Regardless of the initial tread depth, when the tire is new, the tread elements have an initial stiffness. The actual stiffness of the tread elements is dictated by the block size, shape, and the presence of any siping. As the tread is worn, the block height decreases while the tread element stiffness increases. As the tread stiffness increases, some desired tire characteristics, such as wet skid and wet handling, decrease. Hydroplaning characteristics also decrease with increased tread wear due to the reduced groove volume.
Methods have been proposed to maintain a relative tread stiffness as the tire wears. One method is to use split siping, wherein the siping is subdivided below the initial tread surface, as shown in U.S. Pat. Nos. 2,538,491 and 5,316,063. As the tread wears, the single sipe becomes multiple sipes. The increased siping reduces the tread element stiffness.
Disclosed in U.S. Pat. No. 6,408,910 is a method to maintain a desired groove volume wherein the tire is manufactured with molds that create submerged grooves that appear when the tread is worn. However, such a tire is difficult to manufacture due to the necessity of removing the molds that create the submerged grooves.